[DebugInfo] Honour variable fragments in LiveDebugValues

[DebugInfo] Honour variable fragments in LiveDebugValues

This patch makes the LiveDebugValues pass consider fragments when propagating
DBG_VALUE insts between blocks, fixing PR41979. Fragment info for a variable
location is added to the open-ranges key, which allows distinct fragments to be
tracked separately. To handle overlapping fragments things become slightly
funkier. To avoid excessive searching for overlaps in the data-flow part of
LiveDebugValues, this patch:
 * Pre-computes pairings of fragments that overlap, for each DILocalVariable
 * During data-flow, whenever something happens that causes an open range to
   be terminated (via erase), any fragments pre-determined to overlap are
   also terminated.

The effect of which is that when encountering a DBG_VALUE fragment that
overlaps others, the overlapped fragments do not get propagated to other
blocks. We still rely on later location-list building to correctly handle
overlapping fragments within blocks.

It's unclear whether a mixture of DBG_VALUEs with and without fragmented
expressions are legitimate. To avoid suprises, this patch interprets a
DBG_VALUE with no fragment as overlapping any DBG_VALUE _with_ a fragment.

Differential Revision: https://reviews.llvm.org/D62904

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@363256 91177308-0d34-0410-b5e6-96231b3b80d8